A method and a device for controlling a drive unit of a vehicle are described in German Patent Application No. 195 36 038 (U.S. Pat. No. 5,692,472), This publication describes a method for determining a maximum allowable torque of the drive unit as a function of the accelerator position. In addition, the actual torque of the drive unit is determined from operating quantities, compared to the maximum allowable torque, and a torque-reducing intervention into the drive unit made when the actual torque exceeds the maximum allowable torque. In the preferred embodiment of an internal combustion engine, the actual torque is derived from a measured quantity for the air supply to the internal combustion engine. To achieve the object in this manner, torque increases, e.g. through idle speed control, engine losses, and measuring device tolerances, need to be taken into account for determining the allowable torque. The latter must therefore be set to a relatively large value during normal operation. Difficulties, especially with regard to actual torque determination, also arise when using the method in conjunction with direct-injection gasoline engines.
An object of the present invention is to provide features that can be used to achieve more precise monitoring. In addition, this monitoring should be independent of the respective drive engine type.
An acceleration sensor that measures vehicle acceleration is described in the publication entitled xe2x80x9cEinsatzmxc3x6glichkeiten und Zukunftschancen intelligenter Sensoren im Kraftfahrzeugxe2x80x9d (Possible Applications and Future Opportunities for Intelligent Sensors in Motor Vehicles), Bosch Technische Berichte, 1990, Volume 52, pages 30 to 41. A suitable evaluation of the sensor information yields a measurement signal that indicates vehicle acceleration.
A method for estimating the mass of a vehicle on the basis of vehicle acceleration and the driving power, i.e. driving torque, of the drive unit is described in German Patent Application No. 197 28 769.
To monitor drive unit control in a motor vehicle on the basis of the driving torque determined as a function of a signal representing vehicle acceleration, the monitoring function is independent of torque increases caused by operation, which can occur for example in a cold internal combustion engine or when is operating secondary units. The maximum allowable torque can be set more precisely without having to take into account sensor tolerances and operating states in which such torque increases are allowed.
Another advantage is that taking an estimated or measured vehicle mass into account when calculating driving torque from the acceleration signal increases the accuracy of the driving torque value and thus improves monitoring.
One particular advantage is that monitoring on the basis of the driving torque determined from a vehicle acceleration signal is independent of the type of drive unit, so that the monitoring function is suitable for intake manifold injection engines as well as lean-mix or stratified-charge engines, Diesel engines, and electric engines.
The use of an acceleration sensor for measuring acceleration is particularly advantageous because, compared to an acceleration measurement by wheel velocity sensors, it has the advantage that only the acceleration caused by the engine torque is measured when passing over a gradient. Thus, the measured value of the acceleration sensor remains unchanged when a gradient suddenly appears and the engine torque remains constant.
A particular advantage is that, to diagnose the acceleration sensor from the measured acceleration, an engine torque is determined which, during fault-free operation, must lie within a preset tolerance range around the setpoint torque specified by the driver. If not, a latent error has occurred, e.g. the sensor signal present is too small.